"I don't have a choice but to leave the party because the party's policy is way too different from my ideals," he told supporters, according to the Asahi Shimbun.

Hatoyama — who adopted his "Alien" nickname — will explain the reasons for his decision to his constituency in Hokkaido in northern Japan later Wednesday, reports said.

Despite his pledge to step down, Hatoyama's personal wealth may appeal to some of the smaller parties that are springing up around single issues ahead of the poll.

And he has a history of changing his mind: a 2010 pledge to retire was revoked.

Hatoyama became prime minister in 2009 when the DPJ ended half a century of almost unbroken rule by the conservative Liberal Democratic Party.

He was initially seen as the bringer of change, but his reputation took a battering when he reversed course on a plan to remove a controversial U.S. air base in Okinawa.

Having angered voters and provoked Washington in a short space of time, he resigned from the post after just nine months in office, short even by the standards of Japan's revolving-door premiership.

Hatoyama, whose sometimes oddball comments proved a stumbling block for party managers, hails from a powerful political and business clan.

He is a fourth-generation politician and the second in the family to become prime minister, after his grandfather. His other grandfather founded tyremaker Bridgestone.

Hatoyama's wife Miyuki is a former actress turned lifestyle guru with an interest in spirituality. She famously said her soul once visited Venus on a triangular spaceship and that she met Tom Cruise in a previous life.

Opinion polls suggest no one party will achieve a majority in the election and that a possibly shaky coalition is a likely outcome.